Compressive bonded semiconductor elements are widely in use. These semiconductor elements establish electrical and thermal contact without external leads by means of a force applied orthogonal to surfaces of the semiconductor elements to be compression bonded. Manufacturers of compression bonded semiconductor elements specify a nominal desired compressive bonding pressure at which the semiconductor elements are to operate plus a range around the nominal pressure at which each element is designed to operate. The specified pressure varies depending upon the type of the element and its design. Operation outside the specified pressure range can cause circuit malfunction or failure of the compression bonded elements.
Hermetically sealed circuit assemblies having two or more compression bonded circuit elements contained within a hermetically sealed chamber are known. U.S. Pat. No. 3,575,574 discloses individual non-hermetically sealed circuits each containing three compression bonded elements which are contained in a stack having a plurality of the circuit assemblies. The stack is contained in a hermetically sealed chamber. Supports for the individual circuit assemblies do not include any structures for equalizing force between the different columns of compression bonded circuit elements of the stack of circuit assemblies. Furthermore, stacked individual circuit assemblies each having three circuit elements to be compression bonded which are disposed in three columns do not present as serious a problem in achieving specified operating pressures as circuit assemblies having four or more circuit elements to be compression bonded. A universal joint is disclosed in the '574 patent which provides for adjustable movement of heat sinks with respect to the compression bonded circuit element. U.S. Pat. No. 4,313,128 discloses a single hermetically sealed power hybrid circuit assembly. A heat sink to control operating temperature thermally contacts the compression bonded power circuit elements which extends below the bottom wall of a hermetically sealed chamber containing the compression bonded circuit elements. The hermetically sealed semiconductor circuit assembly disclosed in the '128 patent is not designed to be stacked in a stack including multiple circuit assemblies which are clamped together to provide the requisite compressive force for operation. The compressive force in the device disclosed in the '128 patent is established by the bonding of the top and bottom walls of the hermetically sealed chamber through a side member.
Single compression bonded circuit elements in a hermetically sealed chamber are commercially available. These hermetically sealed circuit elements may be stacked and compression bonded by clamping the stack. These elements may have a corrugation in the opposed surface defining the hermetically sealed chamber which provides for relieving stresses due to welding and brazing.
Corrugated bus structures are known to relieve stress. U.S. Pat. No. 4,583,005 discloses a circular corrugated bus and a spider-like conductor which each contain corrugations for relieving thermal stress. A plurality of rings of semiconductor devices are electrically contacted with the circular bus. The individual semiconductor devices are also contacted by the spider-like conductor.
Clamping structures for compressively loading stacked compression bonded semiconductor circuit elements which apply force to a central axis containing the stacked semiconductor circuit elements are known. U.S. Pat. No. 4,504,850 discloses the clamping of a stack of compression bonded circuit elements by application of a force to a convex surface at the axis of the stacked semiconductor circuit elements. Furthermore, swivel, ball and gimbal mounts are known for performing the function of the convex surface to apply uniform force to the central axis of the stacked semiconductor element.
Mechanisms for applying force to columns of stacked compression bonded circuit elements are known. These mechanisms use a force applying member to apply a force to the central axis of the stacked compression bonded semiconductor elements which is sufficient to apply the requisite pressure for normal operation. Bellville washers have been used to apply force to each column of stacked semiconductor circuit elements. See U.S. Pat. No. 3,551,758. Furthermore, coil springs have been used to apply an axial force to each of the columns of stacked semiconductor compression bonded devices by application to an end part of the stack. See U.S. Pat. No. 3,573,574 discussed above. Additionally, it is known to apply compressive force to load stacked compression bonded semiconductor circuit elements by the application of torque to a threaded connecting member which is attached to rigid end plates of the stack. See U.S. Pat. Nos. 3,652,903, 3,936,704 and 4,268,850.
Individual compressed coil springs have been used to load pairs of semiconductor elements to establish desired operating pressure. Opposed faces of the coil spring are in surface contact with first and second compression bonded semiconductor devices. See U.S. Pat. No. 3,921,201.
Compression bonding of a plurality of individual circuit assemblies each having four or more semiconductor elements contained in four or more columns presents special problems in achieving the desired operating pressures in each of the columns. This problem is analogous to leveling a four legged chair in which any variation in the length of one of the legs will cause three of the legs to lie in a single plane with the fourth leg not being in surface contact with the plane containing the three legs unless each of the legs is exactly of the same length. It is for this reason that the establishment of desired operating pressures for compression bonded circuit assemblies are more easily established when only three compression bonded circuit elements disposed in separate columns to be compression bonded are contained in each circuit assembly of a stack of multiple circuit assemblies.